Papermakers conventionally apply coatings to improve the appearance and performance of their paper products, including brightness, gloss, smoothness, opacity and printability. Three main types of mineral pigments have been widely used in coatings in the paper industry: kaolin clay, ground and precipitated calcium carbonate and titanium dioxide. Each type of mineral pigment has its own characteristic properties and brings certain benefits to the paper coatings.
Kaolin clay pigments have been used widely by the paper industry for many years. Perhaps the most common and important kaolin mineral for the paper industry is kaolinite, a hydrous aluminosilicate with a theoretical composition of [Al2Si2O5(OH)4]. The kaolinite structure is composed of a single silicate tetrahedral sheet and a single alumina octahedral sheet arranged so that the tips of the silicate tetrahedrons and one of the layers of alumina octahedral sheet share a common plane. Electron micrographs of well-crystallized kaolinite show hexagonal shaped platy particles, while poorly crystallized kaolinite occurs in less distinct hexagonal shaped particles.
Like other natural mineral pigments, kaolin clays contain some minor impurities, such as TiO2 and Fe2O3, and kaolin clays which contain these impurities generally have a low brightness and an undesirable color. The kaolin industry uses various beneficiation processes to improve the brightness and color of kaolin products. Because of its unique structure and platy particle shape, kaolin clays are widely used to improve runnability of coating formulations and to enhance brightness, gloss, smoothness and printability of a coated sheet.
In recent years, ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC) pigments are finding increasing use by the paper industry. Both GCC and PCC pigments have high brightness and desirable color (blue tint), which also translate into high brightness of coated sheets. However, both GCC and PCC tend to yield lower sheet gloss and poorer printing performance as compared to kaolin clays.
The unique platy shape of kaolin particles enhances sheet gloss, smoothness and printability. However, the platy shape of kaolin particles tend to yield a tight packing structure that is not generally effective for light scattering. To improve light scattering and opacity of the coated sheet, papermakers typically add TiO2 pigments to their coating formulations. Titanium dioxides are highly effective for light scattering because of their high refractive index values (for example, anatase 2.53 and rutile 2.73). However, both forms of TiO2 are expensive.
The kaolin clay industry has developed various technologies to enhance light scattering properties of kaolin clays, and these technologies include:                (1) Engineered pigments produced by mechanically modifying particle size and particle size distribution. This is generally achieved using multiple centrifugation steps. Typically, an engineered clay is made by producing a fine fraction from a particular crude blend. Then the ultrafine particles are reduced by a second centrifugation. The resulting engineered clay pigments have a narrower particle size distribution as compared to that of the feed materials.                    For example, Matthews et al. U.S. Pat. No. 5,168,083 discloses a method of producing a high opacity kaolin pigment by defining an aqueous kaolin slurry via centrifugation to remove a substantial portion of colloidal particles. Prior to the defining step, the aqueous kaolin slurry is mechanically dispersed, ground to break up agglomerates and centrifuged to remove large kaolin particles. The resulting pigments with a narrow particle size distribution yield a coating with special packing characteristics and high porosity. Such a coating is more efficient in light scattering and, therefore, provides improved brightness and opacity to a coated sheet.            The engineered pigments generally perform well in paper coating applications; however, the high production cost and low recovery rate from clay crudes limit their use to high end specialty grades.                        (2) Chemically structured pigments produced by aggregating the kaolin particles through the use of various organic polymers (such as Pratt et al. U.S. Pat. No. 4,738,726), various inorganic compounds (such as Cleland U.S. Pat. No. 4,640,716, Maxwell and Malla U.S. Pat. No. 5,584,925 and Ravishankar U.S. Pat. No. 5,690,728), or inorganic compounds combined with an organic polymer (such as disclosed in Suitch et al. U.S. Pat. No. 5,068,276).                    Pratt et al. U.S. Pat. No. 4,738,726 teaches a high bulking opacifying pigment produced by flocculating hydrous kaolin particles with a minor amount of a cationic polyelectrolyte flocculant, such as a quaternary ammonium polymer salt. There are some limited successes in these technologies for certain specialty grades. This approach is based on the idea to establish and stabilize an open structure for the pigments. Cleland U.S. Pat. No. 4,640,716 teaches using a zirconium ion, such as ammonium zirconium carbonate, to enhance optical properties of the pigment. Maxwell and Malla U.S. Pat. No. 5,584,925 teaches using phosphate compounds to improve the light scattering (opacity), gloss and ink receptivity of the pigment. Ravishankar U.S. Pat. No. 5,690,728 teaches using poly aluminum chloride to produce chemically aggregated pigment.            Suitch et al. U.S. Pat. No. 5,068,276 teaches forming bulked aggregates of mineral particles by adding a polyvalent cation to flocculate the mineral particles and adding polyacrylic acid to cross-link with the polyvalent cation and to cause in situ precipitation of polyacrylate salt on the mineral particle flocs.                        (3) Thermally structured pigments by calcining fine particle size hard kaolin (gray kaolin) crudes. This technology has achieved success in paper filling as well as in paper coating applications.                    Many major kaolin producers produce one or more calcined grades. These calcined products have high brightness and excellent light-scattering properties, and they are widely used by paper makers as an extender or as a replacement for the more expensive TiO2 pigments. Some of these calcined kaolin pigments, such as the product marketed by Thiele Kaolin Company under the trademark KAOCAL, also bring additional benefits to the coated sheet and are used in various proprietary grades and specialty products by papermakers. However, the calcined pigments have some negative attributes, such as abrasiveness and dilatancy (poorer Hercules viscosity) as compared to hydrous kaolin pigments.                        Composite pigments containing at least two pigments from the group that includes kaolin clays, TiO2, precipitated calcium carbonate, talc and other minerals have been developed for improving performance of filler and coating applications. Examples of these composite pigments include:                    A. Composite pigments of calcined kaolin and TiO2 bound together in the form of coherent aggregates using cationic polymers. Nemeh U.S. Pat. No. 5,152,835 teaches binding calcined kaolin and titania pigment using a quarternary ammonium salt polymer. Curtis et al. U.S. Pat. No. 5,336,311 discloses a method for making cationic pigments comprises calcined kaolin and titanium dioxide bounded by a microgel and cationized by epichlorhydrin dimethylamine condensate.            B. Composite pigments of kaolin clay and TiO2 bound together by thermal treatment (calcinations).            C. Composite pigments of hydrous or calcined kaolin and precipitated calcium carbonate (or other metal carbonate) formed via surface coating. Virtanen U.S. Pat. No. 6,143,064 teaches blending an ultra fine precipitated calcium carbonate that ranges in particle size from 30 to 100 nm with another mineral pigment, such as kaolin clay. It is claimed such fine particles of calcium carbonate are attached to the surfaces of pigment particles selected from a group consists of kaolin, calcium carbonate, titanium dioxide and mixtures of such particles.            D. Composite pigments of precipitated calcium carbonate formed in-situ on kaolin clay particles. Bleakely U.S. Pat. No. 6,004,467 teaches a process for aggregating kaolin clay particles by means of precipitation of quicklime (calcium oxide) in the presence of such kaolin clay. However, since kaolin slurry becomes extremely viscous when calcium oxide is added, this approach works only for a slurry at very low solid contents (5 to 7.5% cited in this patent).                            Ravishankar et al. U.S. Pat. No. 6,440,209 improved this process by adding portions of a slaked lime slurry (calcium hydroxide) to the kaolin slurry, followed by carbonation after each slaked lime addition. This improvement allowed a higher solid content of the kaolin slurry at 20%. In addition, this patent revealed that in-situ precipitation of calcium carbonate helped to reduce the content of ultrafine particles without necessitating physical removal of slimes. However, in addition to the desired effect to limit the content of ultrafine particles (such as <0.2 micron), the process in this patent also significantly reduced the contents of fine particles (such as <2 micron and <1 micron). As reported in Table 1 of this patent, the <2 micron particles were reduced from 80% for the Nuclay feed to 61-63% for the resulting composite pigments with 20-30% PCC. The reduction of content of fine particles is undesirable for paper coating applications.                                                
Consequently, there is a need in the industry for a mineral pigment/in-situ precipitated calcium carbonate composite pigment which will provide high brightness, high gloss, high opacity and improved coverage to a coated sheet.